Thermally actuated electric switch of the automatically resetting type



o| 3,361,890 E CTRIC SWITCH OF THE Y RESETTING TYPE Jan. 2, 1968 D. H.

THERMALLY ACTUATED AUTOMATIGALL Filed Feb. 23, 1966 5 Sheets-Sheet 1 i ii I Q fi HHHHHHHHHHH u INVENTOR DONALD H. STOLL, WA. m

ATTORNEY Jan. 2, 1968 H. STOLL. 3,361,890

THERMALLY ACTU T'D ELIE T 16 SWIT OF THE AUTOMATICALLY RE TING T Filed Feb. 25, 1966 5 Sheets-Sheet 2 "c1 I Q L 32 V INVENTOR 44 DONALD H. STOLL, 45 WA. W

ATTORNEY United States Patent 3,361,890 THERMALLY ACTUATED ELECTRIC SWITCH OF THE AUTOMATICALLY RESETTING TYPE Donald H. Stoll, Rock Falls, Ill., assignor to General Electric Company, a corporation of New York Filed Feb. 23, 1966, Ser. No. 529,376 7 Claims. (Cl. 200-139) ABSTRACT OF THE DISCLOSURE Thermally actuated electric switch with a bimetallic member effects flexing movement of a contact between first and second positions in response to a predetermined resetting temperature. A mounting member of the switch has an abutment for limiting movement of the contact beyond the second position, with reduced portions connecting the abutment to a body portion being twisted in the second position for causing torsional movement in response to movement of the abutment toward the mounting member. This establishes an accurate resetting temperature for the switch at the second position.

Background of the invention This invention relates generally to thermally actuated electric switches and more particularly to a thermal switch of the automatically resetting type.

Thermal switches are frequently used to provide thermal over-load protection for electrical apparatus, such as electric motors. One common form of such switch device in use today is shown in Patent No. 2,771,- 528, granted to Don E. Moran which is assigned to the assignee of the present application. The Moran device incorporates a bimetallic member carrying a movable contact which cooperates with a stationary contact, the bimetallic member being so proportioned and arranged as to provide snap-action flexing movement between a normal and second positions. In one direction, the member snaps from a normal position, for example a closed contact position, to a second position, such as an open contact position, in response to a predetermined tripping temperature while it snaps in the opposite direction from the second to the normal positions in response to a predetermined resetting temperature.

In the switch device of the aforementioned Moran patent, the resetting temperature is set by establishing the second or flexed position of the bimetallic member by means of a tab portion on the mounting member for the bimetallic member, the tab portion serving, in effect, as an abutment or stop to limit the flexing movement of the bimetallic member and being manually bent or deformed to establish the second position thereof and thus the resetting temperature.

The resetting temperature of the device of the aforesaid Moran patent is calibrated by placing the device in a controlled temperature and manually deflecting the tab portion until the bimetallic member resets, i.e., snaps back to its normal position. However, when the deflecting force is removed from the tab portion, it tends to spring back toward its original position thus resulting in a resetting temperature which is diflerent from the controlled temperature, the difference between the resultant resetting temperature and the controlled temperature being a function of the amount of spring-back. Thus, the con trolled temperature must be set higher than the desired resetting temperature to compensate for tihs spring-back of the tab portion.

It has been found, however, that in Morans device in which the tab portion is deflected or deformed by bending, there is a considerable variation in the amount of the spring-back of the tab portion depending upon the amount of adjustment required, this variation resulting in a corresponding variation in the calibrated reset temperature. Since the amount of adjustment will vary from one device to the next, it has thus been difficult to set the controlled temperature for proper adjustment of production devices. By way of illustration, in one device having an nominal rating of 11.2 amperes at degrees centigrade and a desired reset or calibrated temperature of 65 C., adjustment of the tab portion was made in a controlled temperature of 78 C. in an attempt to obtain this reset temperature. However, in view of the variation of spring-back of the tab portion from one device to another, the desired reset temperature of 65 C. is diflicult to attain with any degree of regularity. It is therefore desirable to provide an improved thermal switch of the general type shown in the aforesaid Moran patent in which the amount of spring-back of the adjusting tab portion is reduced and more effectively controlled, with a smaller range of variation occurring throughout the range of adjustment between the individual switch devices.

It is accordingly an object of the invention to provide an improved thermal switch of the automatically re setting type.

Another object of the invention is to provide an improved thermal switch of the automatically resetting type in which adjustment of the resetting tmeperature is accomplished with greater accuracy and consistency while retaining economy in its fabrication.

In carrying out the objects in one form, I provide an improved thermal switch device including a snap-acting bimetallic contact-actuating member having a contact movable between a normal and second positions in response to predetermined tripping and resetting temperatures. A mounting member is provided with an adjusting tab portion disposed in the vicinity of one of the positions to limit movement of the contact beyond that position in order to furnish the reset temperature for the device. The tab portion is joined or connected to the body portion of the member by a pair of spaced apart re duced portions so proportioned that these portions become twisted or are placed primarily in torsion as the tab portion is being adjusted to its final adjusted position. This reduces and controls the amount of springback of the tab portion, and makes the spring-back more uniform over the adjustment range which in turn minimizes potential differences in reset temperature between individual switch devices. In addition, the tab is capable of being straightened and re-adjusted many times while the benefits of control of the reset temperature are still obtained. Moreover, the foregoing can be achieved by an arrangment which is also economical to produce in mass production quantities.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of a preferred embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a side, cross-sectional view of a thermal switch incorporating one form of the invention;

assigns FIGURE 2 is a cross-sectional view taken along the line 22 of FIGURE 1;

FIGURE 3 is a cross-sectional view taken along the line 33 of FIGURE 1;

FIGURE 4 is a cross-sectional view taken along the line 4-4 of FIGURE 1;

FIGURE 5 is a fragmentary end view of the mounting member and bimetallic member viewed from the mounting end;

FIGURE 6 is a fragmentary end view of the mounting member, bimetallic member and the stationary switch contact viewed from the adjustment end; and

FIGURE 7. reveals several representative tab forcedeflection and spring-back curves for a given size device constructed in accordance with the present invention and Several representative force-deflection and spring-back curves of prior art devices of the same size.

Referring now to the figures of the drawing, the improved thermal switch incorporating the invention, generally indicated at It), is provided with a cup-shaped housing member 11, preferably formed of suitable insulating material, such as a molded plastic. Housing member 11 has a cavity 12 formed therein which is closed by a pro tective cover member 13.

A mounting member 14 formed of suitable relatively thin sheet metal, for instance, stainless or low carbon steel, is provided positioned in the cavity 12 and having an extension portion 15 extending downwardly from its end 16 through an opening in the bottom wall 17 of the housing member 11, the end 18 of the extension portion 15 which extends outwardly from the bottom wall 17 forming one external terminal for the switch.

Mounting member 14 has a pair of longitudinally extending, transversely spaced apart slots 19, 20 formed in its other end 22, slots 19, 20 thus defining a pair of longitudinally extending, transversely spaced apart arm portions 23, 24 with adjusting tab portion 25 located therebetween. As will be hereinafter more fully described, tab portion 25 is bent'downwardly and has its free end 26 extending upwardly. Arm portions 23, 24 are respectively seated on steps 27, 28 integrally formed on the inner surface of the side wall 29 of the housing member 11 to support tab portion 25 in a preselected location to be brought out in more detail hereinafter.

. A bimetallic member 30 is furnished, preferably having a substantially O-shaped configuration, as best seen in FIGURE 3. A point 32 adjacent the edge of the bimetallic member 30 is secured to indentation 33 in the mounting member 14, adjacent its end 16, as by spot welding, riveting, or the like. An indentation 334 is formed in bimetallic member 30 diametrically opposite from point 32 and extending downwardly toward the bottom wall 17. The indentation 34 pro-stresses the bimetallic member 30 to provide the requisite snap-action in response to temperature or electrical current flow changes. A movable contact 35 is secured to the bottom side of the indentation 34 of bimetallic member 30 in any suitable manner, as by riveting or spot welding.

Mounting member 14 has an opening formed in the center thereof and a threaded opening 36 is formed in bottom wall 17 of housing member 11. A suitable screw 38 is provided seated in the opening in the mounting member 14 with its head engaging the top surface of the mounting member and with its threaded end engaging the threaded opening 36 in the bottom wall 17. A heavy metal 'heat sink member 39, formed of suitable material such as low carbon steel, is provided in the cavity 12 of the housingmember 11 adjacent the bottom wall 17 and having a central opening 46 through which the screw 38 extends. Heat sink member 39 has a projecting portion 42 formed thereon. A terminal member 43 is provided having an upper portion 44 secured to the upper side of projecting portion 42 of the heat sink member 39 and an extension portion 45 which extends downwardly through an opening formed in the bottom wall 17, end 46 of extension portion 55 which projects beyond the bottom wall 17 forming the other external terminal of the switch. Stationary contact 47 is mounted on portion 44 of the terminal member 43 and cooperates with the movable contact 35 on the bimetallic member 30. It will now be seen that a circuit is provided from the external terminal 18 through the bimetallic member 30, movable contact 35, and stationary contact 47 to the other external terminal 46. It will be readily understood that the bimetallic member 3% will flex upwardly with a snap-action thereby to open the contacts 35, 47 in response to a predetermined upper ambient temperature.

In order to render the thermal switch thus far described responsive to excessive current as well as to excessive ambient temperature, a suitable resistance heater 48 is provided in the cavity 12 of housing member 11, disposed between the heat sink member'39 and the bimetallic member 30. Heater 48 has its ends 49, 5t] electrically joined to suitable external terminal members 52, 53 in any suitable manner, as by brazing or soldering, with the external terminal members 52, 53 extending downwardly through the bottom wall 17.

The heat sink member 39 of the device should be of sufiicient mass to store and dissipate heat generated in the heater resistance 48, the heat storage ability of the heat sink member 39 causing the tripping temperature of the device in response to a slow rate of rise of load current appear to be higher than the calibrated tripping temperature. For the device having the 11.2 ampere rating, a mass of three grams was sufl'lcient. Thus, the load current at which the device will trip is higher than it would be without the provision of the heat sink member. This permits a smaller differential between the tripping and the resetting temperatures which in turn allows lower stresses in the bimetallic member and longer life.

In order to provide for deflection of the adjusting tab portion 25 through torsion rather than bending, a transversely extending slot 54 is formed in the mounting plate member 14 spaced from the ends of the slots 19, 20 so as to form a pair of spaced apart, relatively narrow, transversely extending elongated leg portions 55, 56 of reduced cross-section. These portions of reduced cross section integrally connect the base of the tab portion 25 to the arm or body portions 23, 24 of the mounting member and lie in substantially the same plane as the mounting member. A manual adjusting force is applied downwardly on the free end 26 of tab portion 25 and it will be readily seen that such downward force will produce a twisting action and resulting torsional deformation of the connecting leg portions 55, 56 relative to body portions 23, 24. It will be appreciated that portions 55, 56 should be so proportioned to effect this torsional movement. In actual practice, by way of example, satisfactory results were obtained for a given application by forming member 14 of .025 inch low carbon steel material (nominal thickness) and a width of 0.035 inch for each portion 55, 56.

In order to adjust the tripping temperature, the device is placed in a controlled temperature at which the device is required to trip and the screw 38 is then adjusted until the bimetallic member 30 snaps from its normal contactsclosed position as shown in FIGURE 1 to its second flexed and open contact position (shown in phantom in FIGURE 6). Then, in order to adjust the resetting temperature, the device is placed in the controlled temperature at which it is required automatically to reset, with compensation for the spring-back as aforesaid, and the tab portion 25 is deflected downwardly by the application of manual pressure on the free end 26 until the bimetallic member 30 snaps back to its normal contactsclosed position. In one application where the desired trip temperature is C. and the reset temperature 65 C.,

the controlled temperature used for reset adjustment was in the neighborhood of 75 C.

Turning now to FIGURE 7, there is shown two applied force-tab deflection and spring-back curves 57, 57' which are representative of switch devices of a particular size built in accordance with the preferred embodiment as described above. Curves 58, 58' are typical of switch devices of the same particular size constructed with a bent tab of the prior art. All curves are based upon the application of forces to the adjusting tab, with the forces being removed after approximately each 20 mils (0.020 inch) of deflection. The measured amount of spring-back at the various points in the tab adjust range for the devices under consideration is set out in the figure at each point. Curves 57, 57' show a spring-back in the adjustment range for a device incorporating the present invention from 0012-0014 inch while the spring-back of the prior art devices varied from 0.022 to 0.038 inch, depending upon the amount of adjustment produced by the applied force. As will be recognized from a comparison of curves 57, 57 with 58, 58', devices having the present invention evidence both a substantial reduction in the amount of spring-back or tendency to resile and a substantially greater degree of consistency throughout the adjustment range.

Accordingly, it will be understood that while the principles of this invention have been described in connection with an improved thermal switch device, this is intended as an illustrative example of one form of the invention and that the invention is not limited to such example. It is intended, therefore, by the appended claims to cover all modifications that fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thermal switch including a bimetallic contactactuatiug member having means for effecting flexing movement thereof in one direction, from a normal position to a second position, in response to a predetermined tripping temperature and in an opposite direction, from said second position to said normal position, in response to a predetermined resetting temperature; and means for establishing said predetermined resetting temperature comprising a mounting member; abutment means for limiting movement of said bimetallic contact-actuating member in said one direction beyond said second position; at least one transversely extending narrow elongated leg portion lying in substantially the same plane as said mounting member and interconnecting said abutment means and said mounting member whereby said leg portion is torsionally deformed in response to preselected movement of said abutment means toward said bimetallic member thereby establishing said predetermined resetting temperature for said second position.

2. The thermal switch of claim 1 having a resistance heater mounted adjacent said contact actuating member to render the switch responsive to current and temperature, and heat sink means positioned in the vicinity of a major part of said heater and being of suflicient mass for storing and dissipating heat generated in said heater to control the tripping temperature of the switch and to cause a reduction in the diflerential between the tripping and resetting temperatures.

3. A thermal switch including a contact-actuating member having means for effecting flexing movement thereof between first and second positions; a mounting member having a body portion, a tab portion disposed in a final position in the vicinity of said second position for limiting movement of said contact actuating member at said second position, and portions of reduced cross section transversely extending from opposite sides of said tab por. tion and lying in substantially the same plane as said body portion to interconnect said tab portion with said body portion; said portions of reduced cross section being torsionally deformed when said tab portion is set into final position whereby the tendency of said tab portion to resile is reduced.

4. A thermal switch including a bimetallic contact-actuating member having means for effecting flexing movement thereof in one direction, from a normal position to a second position, in response to a predetermined tripping temperature and in an opposite direction, from said second position to said normal position, in response to a predetermined resetting temperature; and means for establishing said predetermined resetting temperature comprising a mounting member including a plate formed of deformable metal and having a pair of elongated longitudinally extending transversely spaced slots defining a pair of longitudinally extending arm portions with a tab portion therebetween; said tab portion having an unsupported end to form abutment means for limiting movement of sadi bimetallic contact-actuating member in said one direction beyond said second position; said plate having a transverse slot formed therein closely spaced from said tab portion and respectively defining, with the ends of the longitudinally extending slots, a pair of reduced portions respectively integrally connecting said tab portion to said arm portion, said reduced portions being torsionally deformed in response to preselected movement of said unsupported end of said tab portion toward said bimetallic member thereby defining said second position for establishing said predetermined resetting temperature.

5. The switch of claim 4 wherein said plate has longitudinally spaced opposite ends, said longitudinally extending slots being deformed in one of said opposite ends, said bimetallic member being secured to said plate adjacent the other opposite end.

6. A thermal switch including a bimetallic contactactuating member having means for effecting flexing movement thereof in one direction, from a normal position to a second position, in response to a predetermined tripping temperature and in an opposite direction, from said second position to said normal position, in response to a predetermined resetting temperature; and means for establishing said predetermined resetting temperature comprising a mounting member; abutment means for limiting movement of said bimetallic contact-actuating member in said one direction beyond said second position; and connecting means of predetermined reduced proportions being twisted in said second position for connecting said abutment means to said mounting member and for causing the connecting means to be placed primarily in torsion in response to preselected movement of said abutment means toward said bimetallic member thereby establishing said predetermined resetting temperature for said second position, said connecting means comprising a relatively narrow member formed of deformable metal and a pair of reduced portions connected at one end to the mounting member and the other end joined to lateral edges of the abutment means with an elongated slot being formed between the reduced portions and the mounting member.

7. A thermal switch including a bimetallic contact-actuating member having means for eifecting flexing movement thereof in one direction, from a normal position to a second position, in response to a predetermined tripping temperature and in an opposite direction, from said second position to said normal position, in response to a predetermined resetting temperature; and means for establishing said predetermined resetting temperature comprising a mounting member; abutment means for limiting movement of said bimetallic contact-actuating member in said one direction beyond said second position; and connecting means of predetermined reduced proportions being twisted in said second position for connecting said abutment means to said mounting member and for causing the connecting means to be placed primarily in torsion in response to preselected movement of said abutment means toward said bimetallic member thereby establishing said predetermined resetting temperature for said second position, said mounting member being formed of relatively thin deformable metal and including an elon- 3,361,890 7 8 gated slot forming an edge of the connecting means and OTHER REFERENCES of the abutment means- 7 Associated Spring Corp., Handbook of Mechanical References Cited Spnng Deslgn page UNITED STATES PATENTS 5 BERNARD A. G ILHEANY, Primary Examiner. 2,771,528 11/1956 Moran 200-422 H. A. LEWITTER, Assistant Examiner. 2,866,039 12/ 1958 Epstein 200122 

